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--- MSOS: Adding print to IMP ---
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Suggested steps to follow in order to extend the oringinal MSOS of IMP in Maude
to include a print statement:

1) Modify imp-msos.maude: Include output.maude at the begining, and 
   add Maude commands for executing sumPrintPgm at the end.

2) Run imp-msos.maude: Everything working before should still work;
   sumPrintPgm gets stuck when the first print statement is encountered.

3) Modify imp-semantics-msos.maude:
   a) Include module OUTPUT in module IMP-CONFIGURATIONS-MSOS;
   b) Add a new attribute, output, to the label record;
   c) Modify the MSOS rule of variable declarations to initialize the state
      to one that also includs the (empty) output buffer.

4) Modify imp-msos.maude: Add the output buffer attribute, holding an empty
   buffer, to any configuration holding an explicit state that appears in any
   of the concrete rewrite or search commands.

5) Run imp-msos.maude: Everything working before should still work, and should
   include and empty output buffer in the result configurations; sumPrintPgm
   still gets stuck when the first print statement is encountered.

6) Add the actual MSOS of print, which consists of two rules.

8) Run imp-msos.maude and check that all programs evaluate properly.


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--- Observations, thoughts ---
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1) We only had to include the output-specific infrastructure and rules,
   without changing any of the existing rules for statements or expressions.

2) We still had to change the rule for programs, to initiate the empty output buffer.
